Referring to FIG. 1, the operation principle of a common scanning apparatus is illustrated. Firstly, the light emitted from a light source 111 is projected onto an object 12 to be scanned. The light reflected from the scanned object is successively reflected by plural mirrors 112, and then focused by a lens 113. The focused light is then imaged onto an image sensor 114 such as a charge couple device (CCD) so as to convert the optical signals into analog electrical signals. Afterwards, the analog electrical signals are converted into digital signals by an analog-digital converter (not shown) to be outputted. In the scanning apparatus 1, the plural reflective mirrors 112, the lens 113 and the image sensor 114 are included in a reading device 11. During operation, the reading device 11 is driven by a driving device (not shown) to scan the object 12. As known, the relative distances between the reflective mirrors 112, the lens 113 and the image sensor 14 have been precisely computed. The distance from the object 12 to the center of the lens 113 is defined as an objective distance. Whereas, the distance between the lens 113 and the image sensor 114 is defined as an image distance. The ratio of the image distance to the objective distance is defined as a magnification of the reading device 11.
In order to precisely image the object 12 onto the image sensor 114, during the process of assembling the reading device 11, the assembler should repeatedly adjust the position of the lens 113 relative to the image sensor 114 in the optical path until the ratio of the image distance to the objective distance (i.e. the magnification of the reading device 11) is desirable. Therefore, it is important to precisely and simply adjust the position of the lens 113 relative to the image sensor 114 during the process of assembling the reading device while maintaining the desired magnification of the reading device 11.